1. Field of the Invention
The present invention relates to a nonaqueous electrolyte battery, a battery pack using the nonaqueous electrolyte battery, and a vehicle having the battery pack mounted thereto.
2. Description of the Related Art
With a rapid propagation of a portable personal computer and a cordless equipment, a secondary battery of a high performance has come to be required as a power source of the portable personal computer and the cordless equipment. A nonaqueous electrolyte secondary battery in which substances capable of absorbing-releasing lithium ions are used for forming the positive electrode and negative electrode has already been developed as such a secondary battery and has already been put to a practical use as a power source of a small electronic equipment. Also, in recent years, the nonaqueous electrolyte battery is used not only in the portable electronic equipment but also in a wide field such as a cordless household electric appliance, a power assist bicycle and a hybrid vehicle.
When the battery is used in such a broadened field, the environment in which the battery is used also differs from that of the battery used in the conventional small electronic equipment. For example, it is conceivable for the vibration to be consecutively applied to the battery in the case where the battery is mounted to a vehicle. It is also conceivable for a large impact to be applied at random to the battery in the case where the battery is mounted to a power tool. The secondary battery is required to withstand such a vibration or an impact and not to get out of order or not to be put in an unsafe state.
On the other hand, the secondary battery is required to exhibit satisfactory battery characteristics. For example, the secondary battery is required to have a large capacity and a high output. In the secondary battery of a large capacity, the weight per battery is increased substantially in proportion to the capacity of the battery. Where the battery weight is increased under the state that the mechanical strength and the rigidity of the constituting factors of the battery are left unchanged, it is apparent that the resistance of the secondary battery to the vibration or impact is lowered. In view of the high output, the electrode terminal of the secondary battery for taking out the current from the battery is required to have a low resistance. In order to satisfy the particular requirement, it is desirable to increase the cross sectional area of the electrode terminal. The electrode terminal having a large cross sectional area is desirable in that the electrode terminal has a high mechanical strength and is unlikely to be bent. On the other hand, the electrode current collector to which the electrode terminal is connected is formed of the metal foil used as a base material of the electrode or formed of a thin metal piece connected to the electrode edge section. Therefore, the electrode current collector has a relatively small mechanical strength in many cases. The electrode current collector having a small mechanical strength tends to be broken with a high probability in the connecting portion between the electrode terminal having a relatively large mechanical strength and the electrode current collector having a relatively small mechanical strength, if displacement is generated in the connecting portion by the external force such as vibration or impact.
In general, in the battery including a flattened electrode group, the restricting force for restricting the displacement of the electrode group is insufficient in a direction parallel to the main surface of the electrode included in the electrode group and, thus, the electrode group tends to be moved inside the case when the battery receives a strong vibration or impact. Particularly, where the case of the battery is formed of a laminate film, it is substantially impossible to suppress the movement of the electrode group within the case. If a space for connecting the electrode current collector to the electrode terminal is formed inside the case formed of the laminate film, the electrode group tends to be moved more easily.
A serious problem that is generated by the movement of the electrode group within the case is an inner short circuit of the battery. Where the electrode group is moved toward the electrode terminal, one edge of the electrode terminal inside the case is stuck against the power generating element (electrode group) so as to bring about an inner short circuit of the battery because the electrode terminal generally has a mechanical strength higher than that of another element, e.g., a separator, included in the electrode group.
Jpn. Pat. Appln. KOKAI No. 2000-215877 given below relates to a flattened battery and is directed to the technique of preventing the electrode lead terminal from being broken when the battery receives vibration or impact. Each of the positive electrode and the negative electrode included in the flattened battery comprises a lead terminal having one edge welded to the terminal section of the electrode and having the other edge portion drawn to the outside of the case. The electrode lead terminal was folded in a manner to cover the terminal section, followed by welding the folded section of the lead terminal to the terminal section. Further, the lead terminal was bent toward the case, with the result that the electrode terminal positioned within the case was shaped substantially like a letter S. In other words, that portion of the lead terminal which extends from the connecting portion at which the lead terminal is connected to the terminal section to reach the case is folded in substantially an S-shape. The moving stress applied to the battery by the impact when the battery is dropped is absorbed or moderated by the elongation or shrinkage of the S-shaped folded portion of the lead terminal so as to prevent the lead terminal from being broken.
However, if the S-shaped folded portion is formed in the lead terminal for moderating the stress, the degree of freedom of the displacement of the lead terminal is increased. Particularly, where the resonance frequency of the lead terminal conforms with the frequency of the external vibration, the amplitude of the displacement of the lead terminal is increased so as to give rise to the metal fatigue in the connecting section between the lead terminal and the current collector or in the lead terminal or the current collector in the vicinity of the connecting section noted above, leading to a breakage problem.
Further, if a substantially S-shaped folded portion is formed in the lead terminal as disclosed in Jpn. Pat. Appln. KOKAI No. 2000-215877 noted above, the movement of the electrode group is promoted, leading to the problem that it is impossible to obtain the effect of suppressing the internal short circuit.